1. Technical Field
The disclosure relates in general to Seebeck/Peltier effect thermoelectric conversion devices and in particular to devices using treated layers of conductive or semiconductive material deposited over a substrate by common planar techniques and with electrical contacts definable with noncritical lithographic or serigraphic techniques.
2. Reference Notions
The Seebeck effect is a thermoelectric phenomenon according to which in a difference of temperature alongwidth an elongated conductor or semiconductor generates electricity. The effect, discovered by the physicist Thomas J. Seebeck in 1821, manifests itself with the presence of a voltage difference at the two ends of a conductive bar subjected to a gradient of temperature ∇T. In a circuit including two junctions kept at different temperatures T1 and T2, between different materials A and B the voltage difference between the two junctions is given by:
                    V        =                              ∫                          T              1                                      T              2                                ⁢                                    [                                                                    S                    B                                    ⁡                                      (                    T                    )                                                  -                                                      S                    A                                    ⁡                                      (                    T                    )                                                              ]                        ⁢                                                  ⁢                          ⅆ              T                                                          (        1        )            where: SA and SB are the Seebeck coefficients (also referred to as thermoelectric powers) relative to the two materials A and B. The voltage values are typically in the order of few μV/K. The Seebeck coefficients depend from the materials, from their absolute temperature and from their structure. The Seebeck effect may be exploited for making devices adapted to measure temperature differences, in terms of voltage differences in a circuit constituted by wires of different materials (thermocouple) or for generating electrical energy (thermopile) by connecting in series a certain number of thermocouples.
From a microscopic point of view, the charge carriers (electrons in metals, electrons and holes in semiconductors, ions in ionic conductors) diffuse when one end of the elongated conductor is at a temperature different from the temperature at the other end. The carriers at higher temperature will diffuse toward the zone at a lower temperature as long as there are different densities of carriers in the portion at lower temperature and in the portion at higher temperature of the elongated conductor. In an isolated system, equilibrium will be reached when, through a diffusion process, heat will become uniformly distributed along the whole conductor. Redistribution of thermal energy due to the movement of charge carriers contemplates a thermocurrent and of course such an electrical current will become null when the temperature of the system becomes uniform. In a system where two junctions are kept at a constant difference of temperature, also the thermocurrent will be constant and therefore a constant flux of charge carriers will be observed. Carrier mobility is reduced by scattering phenomena caused by impurities present in the lattice of the material, by structural defects and by lattice vibrations (phonons). Therefore, the Seebeck coefficient of a material depends in a significative measure from the density of impurities and of crystallographic defects beside from the phonon spectrum in the material. On the other end, locally phonons are not always in thermal equilibrium. On the contrary they move following the temperature gradient and loose energy by interacting with electrons or other carriers, as well as with the lattice defects. If the phonon-electron interaction is predominant, the phonons will tend to push electrons toward a portion of the elongated conductor loosing energy in the process, thus contributing to the electric field in the conductor film. These contribution are ever more important in the temperature range to which the phonon-electron scattering phenomenon is predominant, that is for
                    T        ≈                              1            5                    ⁢                      θ            D                                              (        2        )            where θD is the Debye temperature. At temperatures lower than θD there are fewer phonons that are available for energy transfer while at temperatures above θD they tend to loose energy through a succession of phonon-phonon impacts rather than through repeated phonon-electrons impacts.
It is useful to define a thermoelectric factor of merit of a material as:
                    Z        =                                            S              2                        ⁢            σ                    κ                                    (        3        )            where κ and σ are the heat conductivity and the electrical conductivity, respectively, of the material.
From a technological point of view, the use of Seebeck/Peltier effect thermoelectric converters has being considered of potentially important commercial application. More than half of the heat generated in a thermoelectric power plant is at present dissipated as low enthalpy heat. It is estimated that about 15 millions of megawatt, be dispersed in the process of energy conversion alone. Availability of Seebeck generators capable of converting even only part of such amount of low enthalpy heat in electricity would have a significative positive impact on the energy shortage problem.
Known candidates as thermoelectrically active materials generators have a rather low factor of mint. For example, in case of a thin film of n silicon, doped with 5×1015 atoms of As per cm3, at room temperature, Z≈10−3 K−1. Values of ZT≈1 may be obtained only with costly materials of scarce availability such as Bi2Te3 or alloys of Sb or Se, as an example. In practice, besides few uses at relatively high added value, such as for thermoelectric generation in spacecrafts, the thermoelectric generators based on massive low cost materials achieve conversion yields of the thermal power to electrical power of just about 7%. By comparison, a turbine engine is capable of converting about 20% of the thermal energy to electrical energy.